Vitreous silica has a number of properties that make it a desirable material for many applications. Among these properties are relatively high chemical inertness, low refractive index, and low coefficient of thermal expansion. However, vitreous silica is a refractory material having a working temperature above about 2000.degree. C., which contributes to the difficulty and expense of manufacturing bodies that comprise vitreous silica. It thus would be desirable to have available a glass composition that substantially retains at least most of the desirable attributes of vitreous silica but has a significantly lower working temperature. This application discloses such a glass composition, as well as articles that comprise the glass. Exemplary of these articles is optical fiber.
Silica-based optical fibers have found wide use in telecommunications, and are being considered for such other applications as sensors and optical gyroscopes. As is well known, optical fibers comprise an axially disposed central region (usually referred to as the "core") which has relatively high refractive index and is contactingly surrounded by a region (usually referred to as the "cladding") that has relatively low refractive index. In such a structure, electromagnetic radiation of the appropriate wavelength is guided by means of total internal reflection, with at least a substantial fraction of the total energy being confined to the core, but, at least in the case of single mode optical fiber, a substantial part of the total energy also extending into the cladding region.
Many chemical elements are known to increase the refractive index of fused silica when incorporated therein. These elements, usually referred to as "up-dopants", comprise Ge, Al, P, Ti, Ta, and La. The number of elements which, when incorporated into fused silica, leave the refractive index substantially unchanged or result in a lowering thereof is much smaller. In particular, only F and B are known to be effective down-dopants useful in silica-based optical fibers. Of the above down-dopants, F is finding widespread use, whereas B is used on a much more restricted basis, at least in part due to the fact that the presence of B in or near the core of the optical fiber results in added attenuation at wavelengths above about 1.1 .mu.m. U.S. Pat. No. 4,616,901 (incorporated herein by reference) discloses that the simultaneous presence of P and Al in the core of a silica-based optical fiber permits relatively high Al doping (above about 5%) of the core without devitrification.
As is well known, even so-called single mode optical fiber (i.e., optical fiber which guides only the fundamental mode HE.sub.11) in actuality guides both orthogonal states of the fundamental mode unless special precautions are taken to eliminate mixing between orthogonal states. This is the case in so-called polarization maintaining (PM) fiber. In PM fiber, which typically does not possess the usual circular cross-section symmetry, conditions are arranged such that only one of the two states is guided with relatively low loss. See, for instance, U.S. Pat. Nos. 4,515,436 and 4,529,426, both incorporated herein by reference. PM fibers currently are used primarily for experimental purposes, but are likely to find much wider use in the future, for instance, in sensors and optical gyroscopes and possibly in coherent optical fiber communication systems.
In PM fiber typically one or more stress-producing regions are provided, such that a non-circularly symmetric stress is exerted on the core region, which in turn results in a non-circularly symmetric refractive index in the core. In order to be effective in producing this birefringence, the stress region(s) desirably is(are) close to the core of the fiber. Since fiber material that is close to the core and has a relatively high refractive index may adversely affect the guiding properties of the fiber, it is typically advantageous that the stress-producing region has a refractive index that is equal to or less than that of the cladding. Furthermore, the glass in the stress-producing region typically has to have a coefficient of thermal expansion that substantially differs from that of silica, in order to be effective as a stress producer. The only dopant known to the prior art that both lowers the refractive index of silica and sufficiently increases the coefficient of thermal expansion thereof is boron. Thus, the stress-producing region in prior art PM fiber typically consists substantially of B-doped silica. However, as mentioned above, B-doped silica attenuates radiation of wavelength above about 1.1 .mu.m, requiring a compromise between the conflicting requirements of placement of the stress region close to the core to be effective in stressing the core, and placement of the B-doped region away from the core to avoid excess attenuation, at least for fibers designed to operate at wavelength above about 1.1 .mu.m. As is well known, the currently preferred wavelengths for telecommunications purposes are about 1.3 .mu.m, with about 1.55 .mu.m being likely to become the preferred operating wavelength in the future.
In view of the potential importance of silica-based PM optical fiber, it would be highly desirable to have available means for forming a stress-producing region that has a refractive index that is not substantially larger than that of silica, and that does not result in substantial added absorption of radiation in the wavelength range of current interest for optical communications and other relevant applications, typically from about 0.5 .mu.m to about 1.7 .mu.m. This application discloses such means, and optical fiber comprising such means.